A piston ring is an open-ended or closed ring that fits into a groove on the outer diameter of a piston in a reciprocating engine such as an internal combustion engine or steam engine. The three main functions of piston rings in reciprocating engines are sealing the combustion/expansion chamber, supporting heat transfer from the piston to the cylinder wall., and regulating engine oil consumption. Rings may be gapless or gapped. The gap in a gapped piston ring compresses to a few thousandths of an inch when inside the cylinder bore.
Most automotive pistons have three rings: The top two while also controlling oil are primarily for compression sealing (compression rings); the lower ring is for controlling the supply of oil to the liner which lubricates the piston skirt and the compression rings (oil control rings). Typical compression ring designs will have an essentially rectangular cross section or a keystone cross-section. The periphery will then have either a barrel profile (top compression rings) or a taper form (second compression rings). There are some taper-faced top rings and on some old engines simple plain faced rings were used.
Oil control rings typically are of three types: (1) single piece cast iron, (2) helical spring backed cast iron or steel, or (3) multi-piece steel. The spring backed oil rings and the cast iron oil rings have essentially the same range of peripheral forms which consist of two scraping lands of various detailed form. The multi-piece oil control rings usually consist of two rails or segments (these are thin steel rings) with a spacer expander spring which keeps the two rails apart and provides the radial load.
During certain engine building , rebuilding or repair procedures, the resistance or drag of a piston ring in a cylinder bore is measured. In the typical prior art method, the piston ring is placed on the piston, which is then inserted into the cylinder bore. The piston is then attached to a spring weighing scale and pulled out of the piston. The resistance of the ring is related to the weight measurement on the scale. This can be an inconvenient or ungainly procedure and thus there is need for a fixture which allows a procedure which is more convenient and faster than the method described above.